1. Field of the Invention
This invention relates to examination using ultrasonic waves. More particularly, this invention relates to analysis of contact between an ultrasonic probe and body tissue.
2. Description of the Related Art
Invasive and non-invasive ultrasound techniques have been used to assess tissues within the body. These techniques are particularly relevant to medical procedures in which is necessary to know the relationships of certain tissues to other tissues and to organs that are subject to injury from instruments such as ablation catheters, biopsy needles and the like. For example, cardiac arrhythmias, such as atrial fibrillation, occur when regions of cardiac tissue abnormally conduct electric signals to adjacent tissue, thereby disrupting the normal cardiac cycle and causing asynchronous rhythm.
Procedures for treating arrhythmia include surgically disrupting the origin of the signals causing the arrhythmia, as well as disrupting the conducting pathway for such signals. By selectively ablating cardiac tissue by application of energy via a catheter, it is sometimes possible to cease or modify the propagation of unwanted electrical signals from one portion of the heart to another. The ablation process destroys the unwanted electrical pathways by formation of non-conducting lesions.
A known difficulty in the use of radiofrequency energy for cardiac tissue ablation is controlling local heating of tissue. There are tradeoffs between the desire to create a sufficiently large lesion to effectively ablate an abnormal tissue focus, or block an aberrant conduction pattern, and the undesirable effects of excessive local heating. If the radiofrequency device creates too small a lesion, then the medical procedure could be less effective, or could require too much time. On the other hand, if tissues are heated excessively then there could be local charring effects, coagulum, and or explosive steam pops due to overheating. If the radiofrequency device creates too large a lesion adjacent tissue can be inadvertently ablated. In some cases, perforation of the wall of the heart could occur. It is therefore desirable to know the thickness of the tissue being ablated.
U.S. Pat. No. 8,628,473 to Sliva et al, proposes an ablation catheter comprising an ablation element to ablate a biological member at a target region outside the catheter body and one or more acoustic transducers each configured to direct an acoustic beam toward a respective target ablation region and receive reflection echoes therefrom. The distal member includes a transducer housing in which the acoustic transducers are disposed, the transducer housing including at least one transducer window, which is the only portion in the distal member through which the acoustic beam passes. There is at least the at least one transducer window portion of the distal member.
U.S. Patent Application Publication No. 2013/0123629 by Rosenberg et al. describes a compressive body fat measuring techniques in which a force is applied to the tissue causing narrowing of the adipose tissue layer at the time of measuring. A bias in the adipose layer thickness measurement is dealt with, inter alia, by measuring changes in tissue impedance concurrently or intermittently with ultrasound measurement of adipose tissue layer thickness.
U.S. Patent Application Publication No. 2014/0142438 by Ludwin et al. describes a method, including pressing a distal end of a medical probe against a wall of a body cavity, and receiving from the probe first measurements of a force exerted by the distal end on the wall. The method also includes receiving from the probe second measurements indicating a displacement of the wall in response to the force. The method further includes estimating a thickness of the wall based on the first and the second measurements.